1. Field of the Invention
The present invention relates to systems and methods for balancing the charges of a plurality of batteries.
2. Description of the Related Art
In applications where a plurality of batteries are used as a means of energy storage, imbalances in the states of charge of the batteries can result from a number of causes. For example, batteries can have a "self-discharge" which is a function of the battery temperature, with warmer batteries typically exhibiting greater self-discharge rates than cooler batteries. As a result, warmer batteries will over time exhibit a lower state of charge than cooler batteries. Additional factors, such as manufacturing variation, age of the batteries and others can also have an effect on the self-discharge rate of batteries.
Imbalances in the states of charge of batteries are disadvantageous. Where a battery pack comprising a plurality of series-connected batteries is used, as is typical for example in an electric vehicle, the energy delivery capability of the battery pack is limited by the battery with the lowest state of charge. When that battery is exhausted, the practical capability of the battery pack to continue delivering energy is exhausted. This is true although all of the other batteries in the battery pack may not yet be exhausted. Thus, imbalances in states of charge of batteries impairs the ability of the batteries to deliver energy to their fullest capability.
A number of methods of balancing states of charge of batteries have been proposed. Several of the methods are employed when the batteries are being charged. One such method involves simply charging the batteries with a constant series current until all of the batteries are fully charged. A considerable disadvantage of this method is that the batteries with higher states of charge when the charging process begins will continue receiving current after they are fully charged. This can have a negative effect on battery durability. Also, the heat generated while current is sourced through a fully-charged battery can cause temperature gradients between the batteries in a battery pack. Temperature gradients between batteries are disadvantageous because, for at least the reason discussed above, they promote differing rates of self-discharge between the batteries.
A second charge-balancing method which has been proposed for use while batteries are being charged involves charging the batteries with a relatively high series current until one of the batteries is fully charged. The charging current is then reduced to a trickle current until the remaining batteries are charged. Although this method may help reduce the battery life and temperature gradient problems just discussed, those problems will still be present. Further, reducing the charge current to a trickle can result in a very large charge time for all of the batteries to be fully charged. Particularly in an electric vehicle, short charge times are a very important feature.
A third charge-balancing method which has been proposed for use while batteries are being charged again involves charging the batteries with a relatively high series current until one of the batteries is fully charged. Means are provided to then shunt the charge current around this fully charged battery such that only the other batteries continue to receive charge current. As batteries successively reach full charge, the charge current is shunted around them as well. The charge process is complete when the final battery has reached full charge. U.S. Pat. 3,980,940, issued to Mabuchi et al., discloses such a charge-balancing method. Although this method helps address the disadvantages of the first two methods, it has disadvantages of its own. For example, the shunt components consume energy, so inefficiencies are introduced. Also, shunt components generate heat.
A further disadvantage of any charge-balancing system which operates only when the batteries are being charged should also be noted. Under some operating regimens, the batteries may often not be left "on charge" until the batteries are all fully charged. In such cases, a charge balancing scheme which operates only when the batteries are being charged cannot fully balance the charges of the batteries.
Another charge-balancing method is disclosed in the '940 patent. In the '940 patent, means are provided to discharge all of the batteries to a common point of deep discharge immediately prior to the beginning of battery charging. Although this method may reduce charge imbalances, it also lengthens the total time required for battery charging. This is so because the battery charging event now also includes the discharging which occurs immediately prior to charging. Also, if the battery pack is connected to the charger for only a short period of time, the battery pack may actually have less energy-delivery capability than before being connected to the charger. This would occur because all of the batteries in the battery pack, including the most-discharged battery, would necessarily be discharged to a very low value (below the normal operating range of the batteries) in order to assure balancing prior to charging. A system wherein the batteries can have less energy-delivery capability after a period of time connected to a charger may prove unsatisfactory in the view of a customer.
Thus, a battery charge-balancing method which substantially reduces or avoids the need to provide charge currents to fully-charged batteries; which can be performed energy efficiently and without generating substantial heat; which does not have an adverse impact on battery charging times; which does not depend solely upon the vehicle being "on charge" until all batteries are fully charged for charge balancing to be performed; and which does not discharge the batteries immediately before charging will provide advantages over the prior art.